126 Dr. n. A. Nicholson on Species 



small circular tubes, rendering it minutely porous (PI. XT. 

 fig. 3<7). 



The examples of A. Jrondoso v^h'idi have come under my 

 observation are most commonly attached to the valves of 

 Strophomena aIfcniata,S. planiimhona, and Orthis occidenfalis ; 

 but I have also seen the crusts growing on Chatetes fron- 

 dosus and C. mammxdatus. 



Locality and Formation. — Hudson-River Group (Lower 

 Silurian), Cincinnati and Wayncsville, Ohio. The s])Ocimens 

 liguved are reticulated examples, and arc not so characteristic 

 as the expanded and confluent forms. 



4. Alecto confi(sa, Nicholson. PI. XI. fig. 4. 



Polyzoary adnatc, forming thin crusts which envelop foreign 

 bodies. Cells tubular, multiserial, aiTanged in irregular trans- 

 verse rows, immersed towards their bases, free and elevated 

 above the surface towards their apertures. Cell-mouths cir- 

 cular, terminal, as large as the diameter of the tube, about 

 five in the space of one line. 



All the examples of this species that I have seen are para- 

 sitic upon the columns of Crinoids, which they closely embrace 

 and incrust ; and they might readily be referred to Aidopora, 

 unless care were exercised. The species is nearly allied in 

 essentials to the two preceding, but is distinguished by its 

 constantly forming thin crusts, and by its larger, more closely 

 and irregularly arranged, and more prominent cells. 



Locality and Formation. — Cincinnati Group, Cincinnati, 

 Ohio. Collected by Mr. U. P. James. 



5. Atdopora arachnoideay Hall. PI. XL figs. 5, 5 a. 



Aidopora arachnotdea, Hall, Pal. N. Y. vol. i. p. 76, pi. xxvi. figs. 6 a-6 c. 



Corallum very slender and delicate, attached to the surface 

 of foreign bodies, repeatedly branching, and in many examples 

 anastomosing to form a network. The branches are usually 

 given off at intervals of from one third to two thirds of a line, 

 and are very nan-ow and linear, not exceeding one fifth of a 

 line in width. The corallites have much the form and cha- 

 racter of the cells of the uniserial forms of Alecto, being in- 

 variably arranged in single lines and opening in the axis of 

 the branches. The terminal portion of the corallite is elevated 

 above the surface ; and the calices are circular and not ex- 



f>anded. About four or five calices occupy the space of one 

 ine. 



